Code locks, card readers, or such-to corresponding functions of a visiting preauthorized mobile telephone as disclosed in e.g. WO 00/62521, biometrical readers etc. all have a common disadvantage; in a varying degree they form a bar for occasional and welcome visitors/users (hereinafter “visitors”). This disadvantage is particularly obvious when visiting a tenant or similar in a locked building. Furthermore, the possibility for the visited party to make a “final” judgement/authentification based on from case to case varying situation may not exist. In order to overcome this disadvantage, in particular with regard to tenant buildings, so called entrance telephones, often combined with code locks, have been introduced, which basically can be divided into two groups: A—intercom systems, among which among which also in the trade so called ‘no telephone bill’ systems (e.g. U.S. Pat. No. 3,484,561), which by switching, hardwiring etc. connects to the separate incoming telephone lines of a building at the subscriber side of the Central Office switches, can be counted; and B—“entrance telephone systems” (e.g. SE 7308086-3 and its U.S. counterpart U.S. Pat. No. 3,947,641). “Hybrids” therebetween also exist (e.g. U.S. Pat. No. 5,046,083).
Group A has normally, particularly in larger installations, high complexity and considerable costs for hardware/installation, but involve no running costs as long as the system operates in intended fashion.
Group B has, despite simplicity and low initial costs, another major disadvantage, namely continues running costs, i.e. a building having such an installation involves, apart from payment of a running basic subscriber fee, also pay for completed calls, which on one hand makes it expensive, and on the other is unfair for tenants having few visitors if the running costs are divided equally and charged on top of the rent paid by the tenants.
Various solutions aimed to minimize these problems have been proposed in e.g. SE 9901299-9 and WO 99/44353, each having features that might be of interest for more advanced installations/applications, but less suited for a conventional tenant building and other more common applications.